1. Field of the Invention
The present invention relates to a glass substrate, and more particularly to a glass substrate for use in a magnetic disk.
2. Description of Related Art
Conventionally, as substrates for magnetic disks, there have generally been used aluminum substrates in stationary devices such as desktop computers and servers, and glass substrates in portable devices such as notebook computers and mobile computers. One disadvantage with aluminum substrates is that they are easy to deform and are not hard enough, offering not quite satisfactory smoothness on the substrate surface after polishing. Another disadvantage is that, if a magnetic head happens to touch a magnetic disk, the magnetic film on an aluminum substrate is prone to exfoliate from the substrate. Under this background, it is expected that glass substrates, less prone do deformation, offering better surface smoothness, and affording higher mechanical strength, will be increasingly used not only in portable but also in stationary devices and in other home information appliances.
Generally, a magnetic disk is fabricated by polishing the surface of a glass substrate into a mirror surface, then treating the surface by ultraprecision polishing, and then forming a magnetic layer on the glass substrate by, for example, sputtering. Here, ultraprecision polishing is achieved by, for example, texturing—whereby a pattern of stripes in the shape of concentric circles is formed on the glass substrate surface by polishing using tape—or by mirror polishing—which gives priority to further improving the smoothness on the polished surface while leaving a less conspicuous pattern of stripes. In a magnetically anisotropic recording medium, texturing gives a magnetic disk medium magnetic anisotropy; this improves the magnetic characteristics thereof as a magnetic disk, and also prevents attraction between a magnetic head and the surface of the magnetic disk when a hard disk drive is out of operation. On the other hand, in an isotropic medium such as a perpendicular magnetic disk, mirror polishing greatly improves the smoothness on the substrate surface; this gives the magnetic layer formed on the substrate a fine, uniform structure, and offers greatly improved magnetic recording characteristics. Generally, texturing or mirror polishing of a glass substrate is achieved in the following manner: while a texturing liquid having abrasive particles dispersed in water or a solution containing water as a main ingredient thereof is supplied to the glass substrate surface, polishing cloth or polishing tape of a woven or non-woven fabric or the like is pressed against the glass substrate surface, and the glass substrate is moved.
The recording capacity of a magnetic disk can be increased by reducing the distance between the surface thereof and a magnetic head. Inconveniently, however, with a reduced distance between a magnetic head and the surface of a magnetic disk, if there is an abnormal projection formed on or foreign matter adhered to the surface of a glass substrate, the magnetic head collides with the projection or foreign matter.
Thus, to make it possible to increase the recording capacity of a magnetic disk by reducing the distance from the surface thereof to a magnetic head, it is necessary to eliminate formation of abnormal projections on the surface of a glass substrate, and to form a more uniform and minute pattern of stripes.
In this connection, for example, JP-A-2002-030275 proposes the use of a special texturing liquid for the purpose of forming a uniform and minute pattern of stripes. On the other hand, JP-A-2005-129163, though not directly aiming at improving the effect of ultraprecision polishing, discloses giving the surface layer of a glass substrate a coarse structure with a view to improving the toughness of an information recording glass substrate, alleviating a lowering in strength due to micro-cracks, and reducing the incidence of cracks on collision with a magnetic head.
Inconveniently, however, in actual fabrication, even when glass substrates of the same composition are treated by ultraprecision polishing using the same treatment liquid, the pattern of stripes in the shape of concentric circuits formed on the glass substrates occasionally varies slightly from lot to lot.
Variations in the pattern of stripes on the surface of glass substrates lead to variations in the magnetic characteristics in the magnetic disks formed therewith, and this is undesirable in practical terms.